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Fragile X syndrome

Also known as: Fragile X, FMR1, FRAXA, FXS

Test category:

Familial - Neurological; Paediatric - Congenital disorder; Reproductive - Carrier screen, infertility

Use of test


The fragile X syndrome is the commonest cause of familial intellectual disability. It is caused by a mutation in the FMR1 gene. This is an X-linked disorder, with males often being affected more severely than females.

Carrier women are often intellectually normal; some men can carry the mutation and be intellectually normal. Intellectually normal carriers can develop an ataxic/neurodegenerative disorder in late adult life (both genders) or have premature ovarian failure (women).

This test has diagnostic and familial implications.


In an intellectually disabled person of either gender, an abnormal result is diagnostic of the fragile X syndrome. A normal result essentially excludes this diagnosis; the mutation is found in >99% of patients with this disorder.

In an intellectually normal person, an abnormal result indicates that they are at risk of having affected relatives, and that they may be at increased risk of ataxia or premature menopause in later life.

Ethical considerations:

This is an assay for heritable mutations. It may raise issues of ethics or consent that are different from most other investigations ordered in the routine care of a patient. Use of the Sonic Genetics consent form (below) is not mandated, but could be used to guide the discussion with the patient.

In the case of carrier testing of an unaffected child, national regulations require that the laboratory has evidence of pre-test counselling by a genetics professional and written consent.

Consent form:

Download the Sonic Genetics consent form.


Sizing and triplet repeat primed PCR to detect expansion of the CGG repeat region in the FMR1 gene.

Requesting the test


This test can be requested by any medical practitioner.

Sample required:

4 mL blood in EDTA (separate tube required). Specimens may be collected by the requesting practitioner or at any Sonic Healthcare collection centre (see link below).

To help ensure the quality of the test, a genetic test should be done with a dedicated sample whenever possible i.e. a sample collected specifically for that test rather than a sample that is used for multiple tests.

We recommend that the patient or another adult check the labelling of request forms and sample tubes. 

>Link to Sonic Healthcare collection centres

Special instructions:

Please include statement about family history (or not) of fragile X in clinical details on referral form. If family history is present, please state relationship to patient.

This test is also available as part of our Reproductive carrier screening panel (CF, SMA and fragile X) and Expanded reproductive carrier screen.

Turnaround time:

10 business days.


This test has a Medicare rebate which, subject to the requirements of the Medicare descriptor being met, may cover all or part of the cost. If the patient is not eligible for rebated testing, please contact your local Sonic pathology practice for pricing.


The Medicare details, including descriptor and schedule fee, are listed under MBS item 73300.

Click here for our billing policy.